In recent years, various short-range wireless network communications technologies, notably IEEE 802.11 and Bluetooth, have emerged to enable portable devices (such as laptops, cellular phones, personal digital assistants or PDAs, etc.) to communicate both with each other and with wide-area networking environments. (IEEE 802.11 is a standard of the Institute for Electrical and Electronics Engineers, which was approved in 1997 for wireless Local Area Network, or LAN, signaling and protocols. 802.11 addresses frequency hopping spread spectrum radio, direct sequence spread spectrum radio, and infrared light transmissions. Bluetooth is a specification for short-range wireless connectivity that is aimed at unifying telecommunications and computing. More information on these specifications can be found on the Internet at www.ieee.org and www.bluetooth.com, respectively.)
To enable this communication, various “bridging access points” are being developed. These bridging access points allow a device to wirelessly plug itself into the local LAN. All packets transmitted by the device are then simply forwarded onto the LAN, and the device can read all packets on the LAN (with the access point possibly providing some level of filtering based on, for example, the Media Access Control or MAC address of the device). Examples of commercially available bridging access points include products from Cisco and Lucent (for 802.11) and Widcomm and Axis (for Bluetooth).
Existing short-range wireless LAN solutions have a number of limitations, however. One significant limitation is that seamless roaming is impossible. That is, a device can maintain connectivity while traveling from one access point to another only if the access points are all on the same physical LAN. No existing solutions provide for a device to move seamlessly from one LAN to another without requiring considerable new infrastructure to be deployed (or without requiring significant changes to the device software itself). This is particularly problematic in the wireless environment because users are unlikely to be aware of the physical layout of the LAN topology, and thus they do not realize when they are physically moving outside the range of a particular LAN.
In the Bluetooth environment, roaming through bridging access points is especially troublesome. To maintain connectivity when moving from one access point to another, a device must retain its Internet Protocol (IP) address. A Bluetooth client device, however, obtains IP connectivity by establishing a new Point-to-Point Protocol (PPP) connection with each access point, and the Bluetooth client may therefore request a new IP address (using the Dynamic Host Configuration Protocol, or DHCP). Using bridging access points, obtaining a new IP address for a particular client device each time it moves to a different access point therefore interrupts the device's connectivity and thus makes roaming non-transparent. (PPP is documented in Request For Comments (RFC) 1661, dated July 1994. DHCP is documented in RFC 2131, dated March 1997. Both are available on the Internet at www.ietf.org.)
One solution to the seamless roaming problem involves the use of Mobile IP. In this scheme, the IP address of a mobile device does not change as it moves from one network to another. A device has an associated fixed “home agent” on its home network. When the device moves, it registers with a “foreign agent” on a different network. Messages sent by and destined for the device are tunneled (i.e. forwarded) through the foreign agent. Because the IP address remains static in this configuration, roaming can be achieved. However, this solution has a number of drawbacks. First, it is defined for use only with IP version 4 (“IPv4”) and does not work with IP version 6 (“IPv6”), which is also referred to as “IP Next Generation” and is intended to replace IPv4. Furthermore, a Mobile IP solution requires the LAN administrator to place a foreign agent on each LAN, to assign every user a well-known home agent, and to assign every device a fixed (permanent) IP address. This last requirement is particularly onerous because routable IP addresses are a limited resource on today's Internet; moreover, for security reasons, most systems administrators assign private addresses to internal hosts and hide those addresses from the larger Internet through the use of a firewall that performs Network Address Translation (NAT). Mobile IP also requires considerable effort in order to install and configure a working system.
Another solution to the roaming problem has been proposed by Alex Snoeren and Hari Balakrishnan in their paper, “An End-to-End Approach to Host Mobility,” Proceedings of MobiCom 2000, August 2000. Recognizing the limitations of Mobile IP, these authors suggest that seamless mobility can be achieved by adding additional mechanisms to the Transmission Control Protocol (TCP), allowing an established connection to be “re-mapped” to a client's new IP address. In this way, as the client roams, it is free to obtain a new IP address and consequently re-map all of its open connections. This approach has a number of limitations, however. It requires changes to the TCP implementations on all clients and servers, which is an unlikely occurrence. Applications that are aware of the device's IP address must be modified to learn about and handle the IP address changes that occur as the device roams. The solution does not work for User Datagram Protocol (UDP)/IP-based communication. Finally, the system relies on Dynamic Domain Name Service (DDNS) to allow remote hosts to learn about the client's current IP address; unfortunately, DDNS is not yet fully deployed.
Accordingly, what is needed is a short-range wireless solution that enables seamless network connectivity yet does not suffer from the limitations of prior art techniques.